The present application is related to commonly-owned co-pending application serial number entitled xe2x80x9cCIRCUITS AND METHODS FOR PROVIDING A BANDGAP VOLTAGE REFERENCE USING COMPOSITE RESISTORSxe2x80x9d filed on the same date herewith, which application is incorporated herein by reference in its entirety.
The present invention relates to the field of current reference circuits. In particular, the present invention relates to circuits and methods for providing a current reference with a controlled temperature coefficient using a series composite resistor.
Current reference circuits are used in many CMOS integrated circuits. The purpose of a current reference circuit is to provide a reference current that may be mirrored into other parts of the circuit. The reference current does fluctuate to a certain extent. Such fluctuations are acceptable in some applications, but not in others. Potential causes of reference current fluctuations are operating temperature changes and supply voltage fluctuations.
FIG. 4 illustrates a conventional current reference circuit 400 that provides a relatively stable reference current despite fluctuations in operating temperature and supply voltage. The circuit 400 includes a positive rail VDD and a negative rail VSS. A start up circuit (represented by all the circuitry above brackets 401) includes PMOS transistors 4M1, 4M2, and 4M3 as well as capacitor 402 configured as shown in FIG. 4. The circuitry other than the startup circuit 401 has two stable states upon startup, one that provides a reference current, and one that does not. The startup circuit 401 ensures that the remaining current reference circuitry assumes the stable state that results in a reference current.
Excluding the startup circuit 401, the circuit 400 includes two potential current paths between the positive rail VDD and the negative rail VSS. One current path called a xe2x80x9creference legxe2x80x9d is identified as the circuitry above brackets 403. The current path through the reference leg 403 is through the channel regions of diode-connected PMOS transistors 423 and 427, the channel regions of NMOS transistors 431 and 435, the resistor 439 and the bipolar transistor 441. The other current path called the xe2x80x9cmirror legxe2x80x9d is identified as the circuitry above brackets 404. The current path through the mirror leg 404 is through the channel regions of PMOS transistors 421 and 425, the channel regions of diode-connected NMOS transistors 429 and 433, and the bipolar transistor 437. The total emitter area of the bipolar transistor 441 is greater than the total emitter area of the bipolar transistor 437.
Accordingly, the circuit 400 provides a current along the current path of the reference leg 403 that is relatively insensitive to supply voltage fluctuations. However, due to the temperature sensitivity of the bipolar transistors 437 and 441, the voltage applied across the resistor 439 is largely proportional to absolute temperature. The current in the reference leg 403 may be mirrored into other parts of the circuit (not shown) as needed.
The circuit 400 is similar to early conventional CMOS current reference circuits, except that some conventional current reference circuits use only one PMOS transistor and/or only one NMOS transistor in each leg. However, devices that use more than one PMOS transistor and/or more than one NMOS transistor (called xe2x80x9ccascodedxe2x80x9d devices) reduce the dependency between supply voltage and reference current. Accordingly, cascoded devices provide more stable reference currents.
Another key difference is that in conventional current reference currents, the resistor 439 generally does not have a resistance that compensates for the voltage applied across the resistor being roughly proportional to absolute temperature. Thus, the current reference provided by conventional current reference circuits varies significantly with temperature. In contrast, the circuit 400 reduces the temperature dependency of conventional current reference circuits by providing resistor 439 as a polysilicon resistor which is custom doped so that the resistor 439 (and the corresponding reference leg) has current flowing therethrough during operation that is less dependent upon temperature within a given operating temperature range.
Therefore, the circuit 400 improves upon the prior state of the art by providing a reference current that is less dependent on supply voltage and temperature fluctuations. However, the circuit 400 also requires custom doping which generally introduces manufacturing inefficiencies.
FIG. 5 illustrates another conventional temperature compensated current reference circuit 500. The circuit 500 provides current references that are also stable with temperature and supply voltage fluctuations. However, the circuit 500 does not require the custom doping that is required by the circuit 400.
The circuit 500 is similar to the circuit 400 except that the circuit 500 is not cascoded. In addition, the circuit 500 includes a parallel composite resistor that includes resistor 556 and resistor 558 situated in parallel within the current flow of the reference leg. Standard processing steps may be used to form each of the parallel resistors. By having one resistor be manufactured by process steps that result in a temperature coefficient below a target temperature coefficient, and the other resistor be manufactured by process steps that results in a temperature coefficient above the target temperature coefficient, the sizes of each parallel resistor can be designed so that the parallel composite resistor generally achieves the target temperature coefficient. Thus, the circuit 500 allows for accurate temperature compensation in which the resistors may be formed by standard processes.
Parallel resistors tend to be quite large in order to provide a composite resistor having a given resistance. Accordingly, the parallel composite resistor occupies significant valuable chip space. Therefore, what is desired are circuits and methods for providing a reference current that has a controlled temperature coefficient, which is relatively stable with supply voltage fluctuations, which does not require custom doping, and which efficiently uses chip space.
In accordance with the present invention, circuits and methods for providing a reference current are described. A current reference circuit has a reference leg and a mirror leg configured such that the current in the reference leg is mirrored in the mirror leg and such that a reference current is generated in the reference leg, the reference current being relatively stable with supply voltage and temperature fluctuations. A series composite resistor is disposed in the reference leg. The series composite resistor minimizes space as compared to using a parallel composite resistor. In addition, custom doping is not required to fabricate the series composite resistor.
The current reference circuit is coupled to a high voltage source that is configured to supply a relatively high voltage during operation. In addition, the current reference circuit is coupled to a low voltage source that is configured to supply a relatively low voltage during operation. The current reference circuit defines two current paths between the high voltage source and the low voltage source, one current path being through a reference leg, and the other current path being through a mirror leg.
The reference leg includes a series of MOS transistors including at least one PMOS transistor that is electrically closer in the series to the high voltage source. In addition, the MOS transistors also include at least one NMOS transistor that is electrically closer in the series to the low voltage source.
The reference leg also includes a series composite resistor comprising at least two resistors coupled in series within the current path. The series composite resistor is disposed on either side of the plurality of MOS transistors in series between the high and low voltage sources. The resistors in the series composite resistor may be fabricated by different processing steps. Custom doping need not be employed in fabricating the resistors.
Each resistor in the series composite resistor has its own temperature coefficient. The size of each resistor in the series composite resistor may be designed such that the temperature coefficient of the series composite resistor closely matches the temperature coefficient of the reference current. Thus, the resulting reference current circuit may generate a reference current that is relatively stable with temperature fluctuations. In addition, the series composite resistor may be relatively small compared to a parallel composite resistor for a given resistance.
The reference leg also includes a forward region bipolar transistor coupled in series between the high and low voltage sources. A xe2x80x9cforward regionxe2x80x9d bipolar transistor is defined for purposes of this description and in the claims to be a bipolar transistor that has its emitter-base PN junction forward biased and its collector-base PN junction off during operation. The forward region bipolar transistor is disposed between the series composite resistor and a given voltage source that is either the high or low voltage source.
The mirror leg includes another series of MOS transistors that include at least one PMOS transistor that is electronically closer in the series to the high voltage source. At least one of the PMOS transistors shares a common gate terminal with a PMOS transistor in the reference leg. The MOS transistors also include at least one NMOS transistor that is electronically closer in the series to the low voltage source. At least one of the NMOS transistors shares a common gate terminal with an NMOS transistor in the reference leg.
The mirror leg also includes a forward region bipolar transistor having the same polarity as the bipolar transistor in the reference leg. The forward region bipolar transistor in the mirror leg is disposed between the MOS transistors and the given voltage source. The emitter area of the bipolar transistor in the reference leg is larger than the emitter area of the bipolar transistor in the mirror leg, thereby allowing for a current to flow through the series composite resistors.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.